The present invention relates to a printer and more particularly to a construction of a full-color printer which is reliable without being affected by ambient conditions such as ambient temperature and humidity.
Recently, color reproduction of documents by office automation facilities such as color printers, color scanners, color copiers and so on is being advanced. However, color printers cannot always reproduce an expected color according to the reference data when printing the latter on a sheet of paper. This may be due to two possible reasons. One of the reasons is concerned with the coloring method for color printing. Color printing utilizes the principle of subtractive color mixture, i.e., color is reproduced by superimposing three or four coloring materials (light absorbing pigments or dyes) on an image carrier starting from white (a colorless reflected image carrier), but in many practical cases, the fundamental principle for the above-mentioned subtractive color mixture, i.e., the addition and multiplication effects cannot be realized. To solve this problem, a variety of color correction circuits have been proposed.
The most simple method is to carry out the following 3.times.3 matrix operation according to the input signals "r", "g" and "b" representing red, green and blue respectively. ##EQU1##
In the equation (1), R (red), G (green) and B (blue) are corrected signals and a.sub.11 -a.sub.33 are correction factors. These outputs may be subjected to further gamma correction through a gamma table.
The second possible reason for which the expected color is not always obtained is that the characteristic of the printer changes with elapsed time under the influence of ambient temperature and humidity. For instance, in a sublimation type printer, the printing density is determined according to the temperature of the printing head (i.e., heat energy applied to the printing head). But, since the temperature of the printing head may be affected by the ambient temperature, it is clearly understood that the printing density may vary under the influence of the ambient temperature.
Consequently, a problem has been created where the conventional printer cannot always print out the same data in the same color. Therefore, much time is required for the initial adjustment and maintenance of the printer. This increases the total cost of manufacturing and maintaining the product.
To solve the above-mentioned problem, a printer capable of printing the correct color has been proposed, which has a temperature sensor and controls the amount of energy applied to a printing head by adjusting a constant of a color correction circuit, according to ambient conditions.
The Japanese publication of unexamined patent application JP, A, 2-90142 also discloses a picture image recording device which includes a color printer capable of changing a color correction factor by sensing a variation in the of light's luminosity of the original picture.
Furthermore, TAJIMA Johji, "Color Reproduction Processing for Digital Pagination System," (Journal of Japan Electro-communication Society, 85/4 vol. J68-D No. 4, p.710-717) introduced a color reproduction processing method which is practically applicable in a computer system for making color printing plates. Since in the computer system color images to be printed with cyan (C), magenta (M), yellow (Y) and black (B) inks shall be finally processed on a RGB(Red-Green-Blue) type CRT monitor, the following three kinds of color reproduction problems, i.e., (1) true display of a RGB image, (2) conversion of the RGB color system to the CMYK color system and (3) simulation display of a CMYK image are of great importance. The paper reported that the method could conduct the above-mentioned three kinds of color reproduction processing with a maximal chromatical correctness, obtaining practically satisfactory results.
The above-mentioned method for obtaining the expected colors is such that, the optical density of the print is controlled according to the detected heating temperature of the ink and the illumination of the original picture, which are subsidiary factors not being directly concerned with the printing color's density. The fact that the detected elements are different from the controlled object makes it difficult to design a practical device with an effective controlling system in its operations. Consequently, the prior attempts have not obtained satisfactory results.